1. Field of the Invention
This invention relates to a lightweight lead-acid battery and process delivering high energy densities and high cycle lives suitable for use in electric vehicles. Increasing energy prices and increasing unavailability of hydrocarbon fuels enhance the interest and feasibility of small electric cars. Electric vehicles powered by batteries having energy densities as low as about 20 to 25 W-hr/lb would be satisfactory, provided the batteries have acceptable cycle lifetimes. Lead-acid batteries of the 1920's were capable of delivering energy densities of about 10 W-hr/lb while present day lead batteries in similar applications routinely deliver about 15 W-hr/lb. The theoretical energy density of a lead-acid battery is close to 80 W-hr/lb. Attempts have been made to effect weight reduction in the grids and current collectors, as well as to increase the utilization of active material in the lead-acid batteries.
2. Description of the Prior Art
Lead-acid batteries have been in use for over 100 years with continued efforts toward their improvement. The standard SLI lead rectilinear grid, as described in "Lead-Acid Batteries", H. Bode, New York: John Wiley, 1977, is not suitable for use in electric vehicle batteries due to its high weight and low practical cycle life at high depths of discharge. There have been attempts to replace these lead-alloy grids with lighter metals while making good electrical contact with the positive active material by surface oxide layers as taught by U.S. Pat. No. 4,037,031 and through use of graphite impregnated plastics such as taught by U.S. Pat. No. 4,001,037. There have also been various attempts to prevent shedding of the positive active material, such as encapsulation of the active material within the grid assembly as proposed by U.S. Pat. No. 4,037,030. There have also been proposals to overlay a standard rectilinear lead-alloy grid with plastic net of polyolefin as more fully described by A. Faber, "A New Metal/Plastic Compound-Electrode for Traction Batteries", paper presented at the 11th International Power Sources Symposium, Brighton, Sussex, England, September 1978.
The configurations of the positive electrodes of prior lead-acid batteries utilizing plate-type or tubular-type type structures as more fully taught by "Lead-Acid Batteries", A Reference and Data Book, compiled by the Indian Lead Zinc Information Centre, Elsevier Sequoia S.A., Lausanne, 1977, and "Storage Batteries", G. W. Vinal, John Wiley, New York, 1955, provide limited contact between the positive active material and the current collector. The present invention, through its use of a conductive additive in the positive active material, together with its preferred positive electrode structure, provides greater utilization of the positive active material and facilitates transport between the lead oxide and the current collector. The present invention provides greater utilization of the positive active material than conventional lead oxide pastes as more fully disclosed in "Lead-Acid Batteries", H. Bode, supra.